Once pharmaceutical giant Roche Holding decided to acquire full ownership of biotech firm Genentech, leadership at Roche knew there was no going back. Although the two companies had been working together in some form since the 1980s, and Roche had owned a controlling stake in Genentech since 1990, executives at the Swiss pharma company realized that a failed takeover would permanently poison any future dealings between the two. “If we had gone down the path of increasing the ownership stake … it would have been different,” Steve Krognes, a former Roche executive who is now senior vice president and CFO of Genentech, said during a recent presentation at Wharton San Francisco. “But once the decision was made that we were going to go for full acquisition, and that was announced publicly, there was no way back.” On March 12, 2009, Roche announced a $46.8 billion deal to buy South San Francisco, Calif.-based Genentech. But the path to a merged Roche and Genentech was far from smooth.…